Pupillary Contagion in Infancy

Abstract

Pupillary contagion—responding to pupil size observed in other people with changes in one’s own pupil—has been found in adults and suggests that arousal and other internal states could be transferred across individuals using a subtle physiological cue. Examining this phenomenon developmentally gives insight into its origins and underlying mechanisms, such as whether it is an automatic adaptation already present in infancy. In the current study, 6- and 9-month-olds viewed schematic depictions of eyes with smaller and larger pupils—pairs of concentric circles with smaller and larger black centers—while their own pupil sizes were recorded. Control stimuli were comparable squares. For both age groups, infants’ pupil size was greater when they viewed large-center circles than when they viewed small-center circles, and no differences were found for large-center compared with small-center squares. The findings suggest that infants are sensitive and responsive to subtle cues to other people’s internal states, a mechanism that would be beneficial for early social development.

Keywords

social cognition infancy arousal emotion contagion pupil open materials

An important factor in social-group cohesion is being able to align one’s behavior and emotions with those of other people. One possible mechanism involved in this process is pupillary contagion—a phenomenon in which the pupil size of an observer is influenced by the pupil size of an observed individual. Pupil dilation is not under voluntary control and occurs in response to changes in light as well as to cognitive load, attention, interest, and arousal (Laeng, Sirois, & Gredebäck, 2012). Classic research showing that pupils dilate when people view a person of the opposite sex whose pupils are large was initially interpreted as an arousal response related to perceived attractiveness (Simms, 1967), because faces with larger pupils were also rated as more attractive (Hess, 1965). Recent research also suggests that there may be a role for affiliation in pupillary contagion (e.g., Kret, Fischer, & De Dreu, 2015). However, because pupillary contagion has been found with simple schematic images of circles that only resemble eyes (Hess, 1975), there could also be a low-level, physiological response—such as arousal—that leads to pupillary contagion.

Pupillary contagion has recently been shown to underlie various social-cohesion phenomena in adults. For example, it is stronger within social groups than across social groups (Kret, Tomonaga, & Matsuzawa, 2014) and is related to level of trust in other people (Kret et al., 2015). Pupillary contagion also seems to be related to empathy. In one study, contagion occurred when participants viewed photographs of sad faces, but not when they viewed photographs of faces displaying other emotions (Harrison, Singer, Rotshtein, Dolan, & Critchley, 2006); a related study showed that participants who were more empathic were more likely to make use of pupil size when judging other people’s sadness (Harrison, Wilson, & Critchley, 2007). Together, these studies indicate that pupillary contagion in adults is a social process that can be modulated by empathy or trust but may still be rooted in a more basic, physiological mechanism.

Despite this growing body of research on adults, pupillary contagion has not yet been explored developmentally. A developmental perspective is crucial for examining the origins of and underlying mechanisms involved in pupillary contagion, such as what learning processes or adaptations might be involved in it and how pupillary contagion might change over time from infancy to adulthood. The presence of pupillary contagion early in life could also suggest a role for it in the development of group cohesion, emotion communication, and prosocial behavior.

Could there be pupillary contagion in infancy? Infants demonstrate arousal in response to other people’s arousal from early in life. Specifically, 6- and 12-month-olds seem to show pupillary responses to other infants’ distress and happiness (Geangu, Hauf, Bhardwaj, & Bentz, 2011), and 2-year-olds show pupil dilation when other people are in need of help (Hepach, Vaish, & Tomasello, 2012). Further, newborns cry when they hear another infant’s cry (Dondi, Simion, & Caltran, 1999) and may even automatically mimic emotional facial expressions (Field et al., 1983; Field, Woodson, Greenberg, & Cohen, 1982; but for a critique, see Anisfeld, 1991). Research suggests that the affective-concern aspect of empathy is already present in the first year of life; the more cognitive aspect, such as the ability to help and comfort another, develops later (Davidov, Zahn-Waxler, Roth-Hanania, & Knafo, 2013).

In terms of the basic perception involved in pupillary contagion, infants show consistent preferences for face-like stimuli by the age of 12 weeks (Mondloch et al., 1999), and by the age of 5 to 8 months, they rely on the typical contrast between the sclera and the iris and pupil of the eye when processing faces (Ichikawa, Otsuka, Kanazawa, Yamaguchi, & Kakigi, 2013; Otsuka et al., 2013). However, infants’ face-processing ability (Maurer & Werker, 2014) as well as their understanding of eyes (e.g., Gredebäck, Fikke, & Melinder, 2010; Moore & Corkum, 1998) and emotional expressions (e.g., Leppänen & Nelson, 2009) continue to improve after the age of 6 months. Thus it remains an open question whether subtle depictions of pupil sizes without further social or arousal cues will induce changes in infants’ own pupils and, if so, whether there are developments in this phenomenon over critical months during which face- and emotion-processing abilities are rapidly improving. Demonstrating pupillary contagion in infancy would be strong evidence for its early development as well as one of the first confirmations that infants’ mimicry of other people’s arousal is observable on a physiological level and not just a behavioral level.

To examine pupillary contagion in infants, we chose to test 6- and 9-month-olds given the developments in face and emotion processing occurring in this age range. We also returned to the schematic style of stimuli from Hess (1975), allowing us to display only the most controlled, minimalistic representation of eyes without additional social or emotional information. Specifically, infants’ pupil size was measured as they observed images of pairs of concentric circles with either large or small black centers as well as comparable large- and small-center squares as a control.

Method

Participants

Thirty-two 6-month-olds (18 female; M = 6.16 months, SD = 0.23) and thirty-two 9-month-olds (17 female, M = 8.98 months, SD = 0.29) were included in the analyses. One additional 6-month-old was excluded because of experimenter error, and 2 additional 9-month-olds were excluded because of unsuccessful calibration. Infants were recruited from families that had expressed interest in participating in research studies with their child. Sample size was predetermined by taking into consideration pilot data from adults¹ and that infants would have more missing data as a result of inattention. The procedure was approved by the local ethical review board, and parents of participants provided written informed consent before the experiment began.

Apparatus

Each infant sat on his or her parent’s lap approximately 50 cm from the screen of a remote eye tracker (Tobii T60; Tobii Technology, Inc., Danderyd, Sweden), which was used to record the infant’s eye movements. The Tobii T60 has a freedom of head movement within an area of 40 × 20 × 27 cm. Gaze was recorded at 60 Hz. Before beginning the experiment, a five-point calibration was used, and we required all points to be successfully calibrated.

Stimuli and procedure

Images of horizontal pairs of circles and squares with an outer black border and a congruent central black shape were displayed against a white background. The images were inspired by the circular schematic eyes used by Hess (1975). The outer border was the same size in each circle or square image, but the central shape was one of two sizes, resulting in four total image types that were displayed in one of five locations (i.e., at the center and at each corner of the screen) to avoid visual aftereffects and increase attention to the screen (Fig. 1; see Note 1). More specifically, when shown on the screen, the circles were 5 cm in diameter with a 4.1-cm center (large) or a 3-cm center (small). Squares were 4.3 cm wide with a 3.5-cm center (large) or a 2.6-cm center (small). The outer border was 0.1 cm thick for both shapes. This resulted in large-center shapes that were approximately 75% black and small-center shapes that were approximately 45% black.

Fig. 1.

Examples of stimuli. In each trial, a pair of circles or squares that had large centers or small centers was presented on the screen in one of five locations (i.e., center, top left, bottom left, top right, or bottom right).

The infants viewed each image type 12 times, for a total of 48 trials. Images were presented in one of four counterbalanced orders. Two images of the same type (e.g., large-center circles) were never seen more than twice in a row, and images were never presented in the same location twice in a row. Each image was shown for 5 s. Before each image, the infants first saw a brief, attention-grabbing animation in the center of the screen, followed by 1 s of blank white screen. After every seven images, a longer (6–9 s) animated video was shown. None of the attention-grabbing animations showed face- or eye-like images of any kind so that we could avoid priming the infants. Throughout the presentation, neutral instrumental music was played to increase attention. The total presentation lasted approximately 9 min.

Data reduction and analyses

Data files exported from the Tobii eye tracker were processed to determine pupil measurements per trial for each infant. To process the time series data, we used Time Studio (Version 3.15; timestudioproject.com; Nyström, Falck-Ytter, & Gredebäck, 2015; the exact analysis used in this study can be downloaded using uwid ts-0ee-ea4 from within the Time Studio program), a MATLAB-based open-access analysis tool. First, data gaps of up to 10 samples were interpolated linearly, and the data were smoothed using a moving average over 5 samples. The pupil-size values for each trial were then adjusted using the first 0.5 s of the image display as a baseline; the pupil size at baseline was subtracted from the average pupil size from the analysis period of 0.5 to 2.5 s. To accommodate variation in the pupil as a result of differing initial light responses, we set the end of the baseline to the approximate halfway point between the maximum and minimum points on the pupillary light-reflex response—the initial dilation and contraction in pupil size that occurs when any new stimulus is presented to help the eye adjust to the light (Nyström, Gredebäck, Bölte, & Falck-Ytter, 2015; see Fig. 2b). Given the novel nature of the study, we selected the analysis period by first visually inspecting the pupil data. We observed that the clearest differentiation between the image types seemed to occur within the first half of the 5-s presentation. However, we also performed an additional analysis using the period from .5 s to the end of the 5-s trial (see the Supplemental Analysis in the Supplemental Material available online). Trials that were missing more than 50% of sampled data because of either technical problems or inattention were excluded (42.7% of trials; 58.8% of these had no recorded data at all). Finally, trials with a score more than 3 standard deviations from the mean were removed (1% of remaining trials).

Fig. 2.

Pupil-size results. The mean difference in baseline-adjusted pupil size (a) is graphed as a function of shape. The difference was calculated by subtracting pupil size for small-center shapes from that for large-center shapes, and the data are collapsed across age groups. Error bars indicate ±1 SE. The asterisk indicates a significant difference from zero (p < .001). Mean raw change in pupil size (b) is plotted as a function of time for circles (top) and squares (bottom). Shaded regions indicate ±1 SE.

Statistical analyses were performed using the package lme4 (Version 1.1-8; Bates & Sarkar, 2007) for the R software environment (Version 3.1.1; R Development Core Team, 2014). Data were analyzed using mixed-effect regression models that included individual pupil responses per trial as the dependent variable and participant as a random effect. Random effects are beneficial for taking into account the individual variability of participants to strengthen analyses (Baayen, Davidson, & Bates, 2008). Significant fixed effects were determined by comparing models with and without the fixed effect to test whether it was beneficial for explaining the dependent variable.

Results

Preliminary analyses indicated that the location of the image on the screen was a significant fixed effect for pupil size, model comparison χ²(5) = 285.93, p < .001. Location was not a variable of interest in the study, and pupil size is known to vary according to the screen location of a stimulus (Brisson et al., 2013); therefore, we normed the data using adjustment factors for each location (the mean for a given location minus the overall mean). These adjustment factors were subtracted from each individual trial value in that location. There was no effect of participant sex on pupil size, and there were no interactions between sex and center size or image shape, so data were collapsed across sexes.

An initial analysis to examine pupillary contagion included the fixed-effect factors of age group (6 months old or 9 months old), center size (large center or small center), image shape (circle or square), and their interactions. This analysis revealed an interaction between center size and image shape, b = 0.052, SE = 0.019, 95% confidence interval (CI) = [0.015, 0.089]; model comparison χ²(1) = 7.731, p = .005, so data from the two image shapes were analyzed separately using the fixed-effect factors of center size, age, and their interaction.

As predicted, there was an effect of center size for circles, b = 0.048, SE = 0.013, 95% CI = [0.022, 0.073], model comparison χ²(1) = 12.941, p < .001: Infants displayed greater pupil dilation in response to large-center images (M = −0.154, SD = 0.196), 95% CI = [−0.172, −0.135]) than in response to small-center images (M = −0.200, SD = 0.212, 95% CI = [−0.220, −0.180]). There was no effect of age on pupil size. For squares, neither age nor center size were significantly related to pupil size (ps > .10; see Fig. 2).

A secondary analysis was performed to examine the effect of image shape for each center size. For large-center images, there were no effects of age, shape, or their interaction (all ps > .10), but for small-center images, there was a significant effect of shape, b = 0.062, SE = 0.014, 95% CI = [0.035, 0.088], model comparison χ²(1) = 20.233, p < .001: Infants displayed greater pupil dilation in response to squares (M = −0.136, SD = 0.220, 95% CI = [−0.156, −0.115]) than in response to circles (M = −0.200, SD = 0.212, 95% CI = [−0.220, −0.180]).

Discussion

The current results revealed that both 6- and 9-month-old infants were sensitive to apparent pupil size in schematic images, and they reacted with pupil size changes of their own. The effect was not based simply on adjustment to light, because there was no comparable effect for images of squares. One explanation for this effect is that infants sensed a subtle cue that indicated another person’s arousal; the infants’ sensitivity to such arousal then induced changes in their own arousal levels and thus pupil size. A second possible explanation is that viewing larger pupils triggered social-cognitive processes, such as liking or empathy, which then led to pupillary contagion. In either case, the findings suggest an early-developing sensitivity to other people’s pupil size that influences infants’ own physiological state and is not reliant on infants’ increasing face- and emotion-processing skills between the ages of 6 and 9 months.

The current study is one of the first to use pupil size as an outcome measure when examining infants’ responses to depictions of arousal. Geangu and colleagues (2011) demonstrated that 6- and 12-month-olds had greater pupil dilation while viewing videos of other infants laughing and crying than while viewing videos of neutral babbling. However, there was only one video per emotion, and additional differences across the videos (in motion, contrast, volume, etc.) could have contributed to arousal and thus to pupil-size differences. In addition, infants looked at the face of the infant in the video less often in the neutral condition, which could further contribute to pupil differences. Thus, our study is the first with controlled stimuli to demonstrate pupil-size differences in infants in response to images that suggest arousal.

An implication of the current findings is that pupillary contagion could be fundamental for empathy development. Research with adults on pupillary contagion has already begun to suggest a link to empathy (Harrison et al., 2006, 2007), and contagious arousal would be a tool for young infants to recognize and share other people’s emotional states—a fundamental aspect of empathy (Hatfield, Cacioppo, & Rapson, 1994). It is interesting to note that research on pupillary contagion in adults is increasingly demonstrating its specificity (e.g., within social groups; Kret et al., 2014), whereas our results with schematic images suggest that a low-level, general mechanism could be at work. It is possible that pupillary contagion occurs automatically unless other available information indicates that it is not important to share arousal in a particular situation, resulting in top-down modulation of the effect. For now, whether infants also modulate pupillary contagion on the basis of other social information remains unknown, but the current data provide evidence that the general mechanism is in place from early in life and has the possibility to be shaped further with development.

An additional finding in the study was that squares overall led to as much pupil dilation as large-center circles. This result is in line with findings indicating that sharp angles are more arousing than round shapes, as indicated by amygdala activation in adults (Bar & Neta, 2007). This finding for pupil dilation in response to squares also rules out the possible confound that viewing squares somehow hindered pupil dilation.

The current data cannot conclusively determine the mechanism behind pupillary contagion. One possibility is that pupil-size correspondence is based on automatic mimicry caused by mirroring processes, without the involvement of arousal or with arousal resulting only from biofeedback processes after pupil dilation, as has been suggested for emotional expressions (e.g., Carr, Iacoboni, Dubeau, Mazziotta, & Lenzi, 2003). As yet, however, there is no evidence that mirror neurons are involved in autonomic activity, such as pupil dilation, making this an unlikely explanation. A second possibility is that infants’ pupil dilation is related to interest or liking because the images that depict large pupils are more appealing or interesting to them (e.g., because they are similar to the pupils of a caregiver who looks affectionately at the infant).

A third possibility is that infants’ pupil dilation results from arousal because they have learned to associate larger pupils with other more perceptible arousal cues (e.g., facial expression and vocalizations) in other people that lead to arousal and thus pupil dilation in themselves, generating the contagion response. We argue that pupil size, given its subtlety when viewed in daily life, is unlikely to be salient enough for infants to form associations unless they are predisposed to be particularly sensitive to pupil size. That is, we propose that pupillary contagion results from an adaptation that allows infants to take advantage of available signals to synchronize their internal states with those of other people; this proposition is in line with the perception-action model of empathy (Preston & de Waal, 2002). Specifically, detection of dilated pupils in another person leads to a change in arousal, liking, or interest in the observer, which results in dilated pupils—an outward sign of the state two people have begun to share.

One limitation of the current study was that pupil dilation was our only measure, making it difficult to say for certain whether infants’ reactions stemmed from arousal. In previous work with adults, controlling for heart rate did not completely explain the pupillary-contagion effect (Kret et al., 2015). However, different arousal measures capture different aspects of arousal (e.g., Bradley, Miccoli, Escrig, & Lang, 2008), so controlling for heart rate does not necessarily rule out arousal as a mechanism behind pupillary contagion.

A further limitation is that the schematic eyes and pupils might not have been perceived as eyes by infants and that the pattern of responses simply reflects the fact that pupils can mimic the shape and size of circles but not squares, without any involvement of arousal. However, given previous findings on infants’ early sensitivity to schematic faces (Mondloch et al., 1999), eye contrast (Ichikawa et al., 2013; Otsuka et al., 2013), and eye direction (Farroni, Csibra, Simion, & Johnson, 2002), it seems likely that the images did suggest eyes to the infants.

The current study is the first evidence of pupillary contagion in infants. We demonstrate that 6- and 9-month-olds’ reactions to a subtle cue of other people’s internal states occur on a physiological level. Given the early age at which pupillary contagion has been observed, it could function as a way to communicate and align internal states across conspecifics early in life. Furthermore, this spontaneous transfer of internal states could be fundamental for social group cohesion and the development of empathy.

Footnotes

Action Editor

Eddie Harmon-Jones served as action editor for this article.

Funding

This research was supported by the European Research Council (ERC-StG CACTUS 312292).

Declaration of Conflicting Interests

The authors declared that they had no conflicts of interest with respect to their authorship or the publication of this article.

Supplemental Material

Additional supporting information can be found at

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All materials have been made publicly available via Databrary and can be accessed at https://nyu.databrary.org/volume/214. The complete Open Practices Disclosure for this article can be found at http://pss.sagepub.com/content/by/supplemental-data. This article has received the badge for Open Materials. More information about the Open Practices badges can be found at https://osf.io/tvyxz/wiki/1.%20View%20the%20Badges/ and .

Notes

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